The present invention relates to ultrasonic testing of sheet metal or plates by means of continuous ultrasonic waves traversing the sheet or plate.
Metal sheets or plates are, for example, tested by means of utrasonic waves applied by a transmitting transducer which is coupled to the surface of the test object by means of water, such as a water jet. The ultrasonic wave traverses the material and is picked up at another location by means of a transducer which is also coupled to the test object by means of water. A plurality of transmitting transducers may be coupled to one side of a metal sheet or plate stock and receiving transducers to the other side.
By way of example, a transmitting transducer emits a cw beam of 2 megahertz. In order to avoid standing waves, this wave serves as a carrier and is modulated to have sidebands (shifts) of .+-.50 kHz and an additional frequency modulation of, e.g., 200 Hz. It was found, however, that frequently the resulting test sensitivity is not sufficient to fulfill present-day requirements in respect to flaw and defect detection.
Another problem is the following. The transmitting transducer is, for example, controlled via a passive network through which the energizing voltage is applied. Tolerances in electric properties of the transducer head, such as input impedance, resonance frequency, and so forth, produce certain mismatching conditions. In the case of a test device using a plurality of such heads (test channels), one obtains different sensitivities in the different channels. Particularly, one channel may recognize a defect that was missed by another one, so that an uncertainty is introduced into the overall evaluation, being based on processing of the various channels, and designed to verify defect detection.
Known ultrasonic testing devices include linear amplifiers for responding to the signals received by the receiving transducers of the system. These amplifiers are adjusted with respect to sensitivity without regard to an absolute level of sensitivity which the system as a whole may achieve. This may render individual ones of the test channels more sensitive to external interference because too high a sensitive system is prone to indicate defects where there is only "noise."